The evolutionary genetics of insertion sequences in several species of enteric bacteria will be determined by means of DNA sequencing of the elements. The insertion sequences of interest are IS1, IS3 and IS30, chosen because of previous studies of their distribution and abundance showing that they represent the three main types of insertion sequences in terms of the strength of regulation of transposition. The organisms to be studied include the three Types (races) of Escherichia coli represented in the ECOR collection of reference strains, and Shigella sonnei, E. hermannii, E. vulneris, and E. blattae. E. fergusonii will also be examined, in case any of the other species should lack any of the insertion sequences. We will also sequence a plasmid-borne copy of each element, and two copies of apparently genetically divergent elements which hybridize weakly with probe DNA in Southern blots. From these data we will determine the noncoding and possibly regulatory sequences that are conserved in the IS elements, the conserved open reading frames, the rates of evolution of inverted repeat sequences as compared with nonsynonymous and synonymous substitutions, and the rates of evolution of the three insertion sequences as compared with each other and that of the chromosome as a whole. The evolutionary history of each insertion sequence will be determined by parsimony methods and compared with the species phylogeny.